This disclosure generally relates to the stabilization of dihydric phenols. More particularly the disclosure relates to stabilizing the dihydric phenols, under the conditions prevailing in a polymerization reaction.
Polycarbonates are ranked among the most important of the world's engineering thermoplastics. Bisphenol A polycarbonate is currently the most widely used polycarbonate and its world wide annual production exceeds one billion pounds. Polycarbonates are used in hundreds of applications such as eyeglass lenses and optical media, where their transparency and tough physical properties are beneficial. Some dihydric phenols, for example methyl hydroquinone, are key monomers for preparing polycarbonates that are used in specialty applications, such as, for example, in packaging for cosmetic, perfume, or biochemical applications. In addition to being chemically resistant and transparent, the polycarbonates also need to have improved color (i.e., reduced yellowness index).
However, some dihydric phenols, such as, for example, hydroquinone and methyl hydroquinone, acquire color under oxygen, heat or storage, either due to oxidation and/or due to thermally activated processes. This discoloration is more intense in the presence of iron and/or other transition or non-transition metals, which are known to form colored metal complexes with the dihydric phenols. This discoloration may cause quality issues in the production of the dihydric phenols and in the production of polycarbonates made using these dihydric phenols. The discoloration also lowers the commercial value of the dihydric phenols and the polycarbonates made therefrom.
In order to prevent or minimize the discoloration of dihydric phenols, it is known to mix the dihydric phenol with a suitable stabilizer. However, the effective quantity and the compatibility of the stabilizer depends very much on the process conditions used for making the dihydric phenols and the corresponding polycarbonates, the level of trace metals already present in the dihydric phenols and whether the dihydric phenol is exposed to metal parts or containers during storage or during the preparation of the polycarbonates. Further, the stabilizers known in the art are low boilers or decompose at high temperatures and release corrosive chemicals causing discoloration of dihydric phenols either in contact with air or metal. It is not economically feasible in the process to have completely metal-free dihydric phenols. It is also not economically viable to use an all glass assembly to avoid contact with metal during storage or during the polymerization reactions.
Hence there is a need for suitable stabilizers that will prevent or minimize the discoloration of dihydric phenols, especially under the high temperature conditions in which the polymerization reactions are conducted.